This invention is directed to antibiotics and, more particularly, to a process for preparing an antibiotic known as cirramycin A.sub.1. Another aspect of the invention is a thermo-stable incubation substrate for the conversion of cirramycin B.sub.1 to cirramycin A.sub.1.
The antibiotic known as cirramycin and its preparation is disclosed in U.S. Pat. No. 3,159,540. This antibiotic which comprises components identified as cirramycin A and cirramycin B can be prepared by fermentation with an organism designated Streptomyces cirratus (ATCC 14699). It has been found that the antibiotic cirramycin contains not only A and B components, but can also be separated into a series of components designated as B.sub.1, B.sub.2, B.sub.3, A.sub.1, A.sub.2, A.sub.3, A.sub.4, and A.sub.5.
It is also known that mild acid hydrolysis, with 0.1 normal hydrochloric acid, of cirramycin B.sub.1 gives cirramycin A.sub.1. See T. Suzuki, "Bull. Chem. Soc.," Japan, 43, 292 (1970). Conversion by this method is not satisfactory, because it is characterized by significant loss of biological activity and very low yields.
However, it has now been discovered that high yields of high-activity cirramycin A.sub.1 can be obtained in good yields by incubating cirramycin B.sub.1 in a substrate comprising an aqueous solution of cane molasses. Good yields of cirramycin A.sub.1 can be achieved by this process which comprises admixing cirramycin A.sub.1 with a solution of cane molasses and incubating the admixture until a suitable degree of conversion is achieved. The conversion of B.sub.1 to A.sub.1 can be followed by periodically withdrawing and assaying samples.
Although the amount of cane molasses in the substrate is not narrowly critical, it is generally preferred to employ an aqueous solution of relatively low molasses concentration as a substrate for the incubation. For example, a concentration of about 2 percent cane molasses has been found to provide good yields of high activity cirramycin A.sub.1 when used as a substrate in connection with an incubation of cirramycin B.sub.1 at about 37.degree. C. It will be appreciated that variations in the concentration of cirramycin B.sub.1 being incubated may give rise to variations in the substrate concentration for optimum results.
In practice, the process of this invention comprises the steps of preparing cirramycin B.sub.1 by fermentation of a suitable strain of Streptomyces cirratus. It is preferred that a strain capable of producing high yields of cirramycin B.sub.1 be utilized in conjunction with standard fermentation techniques. The cirramycin B.sub.1 is then extracted from the fermentation broth by solvent separation procedures to provide an aqueous extract containing the cirramycin B.sub.1.
In practice, the cirramycin B.sub.1 can be further separated from the aqueous extract and used as a solid, or the cirramycin B.sub.1 -containing extract can be added directly to the cane molasses substrate and incubated. After completion of the conversion, cirramycin A.sub.1 can be recovered by solvent extraction. Minor components which are not converted to A.sub.1 are taken into an organic solvent phase, and cirramycin A.sub.1 is recovered from the aqueous phases and can be, if desired, further purified by countercurrent distribution.